Platelet activation begins with the initial deposition of platelets on a damaged vessel wall, then continues as additional platelets are recruited and adhere to each other. These events bring platelets into stable contact with each other, forming junctions where protein:protein interactions can occur between adjacent platelets. The long term goal of this project is to understand how events at platelet junctions contribute to the platelet response to injury. Our hypothesis is that 1) relevant signaling continues after platelet aggregation has begun, 2) some of the signaling arises from interactions between molecules other than integrins on the surface of adjacent platelets, and 3) these contact-dependent interactions at junctions can serve either as positive regulators, promoting the growth and stability of the hemostatic mass to prevent re-bleeding, or as negative regulators, limiting growth and stability so that vascular occlusion is avoided. During the most recent funding period we have identified ephrin B1 and semaphorin 4D on the platelet surface, and shown that the binding of these ligands to their respective receptors (EphB1 and EphA4 for ephrinBI; CD72 and plexin B1 for sema4D) promotes thrombus growth. We have also determined that ESAM, a putative cell adhesion molecule in the CTX family, translocates to junctions when platelets are activated and then acts as a negative regulator, so that loss of ESAM expression promotes, rather than impairs, extension of the platelet mass. The studies described in this proposal are divided into four specific aims focusing on platelet junctions and contact-dependent interactions. Aim #1 will test our current model that ESAM is a negative regulator of platelet:platelet interactions and explore the consequences of a loss of ESAM function on platelet activation using an existing line of ESAM knockout mice. Aim #2 will focus on the molecular basis for ESAM's contribution, starting with our recent identification of two scaffold proteins, NHERF-1 and CAL, that bind to the ESAM cytoplasmic domain. Aim #3 is a comparative analysis of the three other CTX family members expressed in platelets (JAM-A, JAM-C and CD226) to determine whether their role is the same as ESAM. Initial results obtained with JAM-A knockout mice, suggest that this may be the case. Aim #4 is devoted to the characterization of additional junction molecules in platelets, starting with ephrin A1 on platelets and continuing with an unbiased search for novel proteins.